A phrase used to determine the appropriate circumstances under which to flush a toilet. Urine was to be left unflushed in the toilet bowl while feces were to be flushed right after bowel evacuation. This was used in efforts of water conservation either in the sense of environmental conservation or the saving of clean toilet tank water during power outages in remote areas.

“Man, don’t waste three gallons of water to get rid of a cup of sterile piss.”
“Why is that kindly hippy?”
“Just remember man, 'If it’s yellow, let it mellow. If it’s brown flush it down.'”
“Thank you, sir. Perhaps with the water we save in the crapper you might be able to bathe more thoroughly in the future.”